Elements mounted on a substrate of a composite integrated circuit (IC) device include a heater element, such as a resistor, which heats under a load of high electric current, and a temperature-restricted element, such as a semiconductor, which has restriction on operating temperature. Heat generated from the heater element adversely affects the temperature-restricted element. Therefore, in a composite IC device, a dissipating member is attached to the temperature-restricted element for enhancing heat dissipation, or the heat element and temperature-restricted element are respectively mounted on separated substrates.
Recent downsizing requirement causes a composite IC device to include a heater element and a temperature-restricted element on its single substrate. For instance, for downsizing an electronic control unit (ECU) as a vehicular composite IC device, a circuit driving a motor or lamps is included in the single circuit of the ECU. FIGS. 9A to 9C show an instance of an ECU of this vehicular composite IC device 100 whose substrate includes a lamp driving circuit. FIG. 9A shows a schematic diagram of an electric circuit. FIG. 9B shows a top view of the composite IC device 100, while FIG. 9C shows a sectional view taken along line 9C—9C.
A lamp driving circuit within the ECU 100 is surrounded by a dashed line in FIG. 9B and constructed of a power transistor 51 and a resistor 41. Within the ECU 100, a control circuit (not shown) including a logic IC or memory IC is contained. The lamp 91 is connected with a battery 92 outside the ECU 100 and also with the power transistor 51 via the resistor 41. An electric current flowing through the power transistor 51 is controlled by a signal from the control circuit for driving the lamp 91.
The power transistor 51 and the resistor 41, both of which a large electric current flows through, heat up during the operating, so that they are categorized as heater elements. A typical permissible temperature of the resistor 41 is 300 to 500° C. and has a margin against an operating temperature. The resistor 41 is thereby not categorized as a temperature-resisted element. By contrast, the power transistor 51 of a semiconductor has a permissible temperature of less than 150° C. to be thereby categorized as a temperature-restricted element. Furthermore, a semiconductor of a logic IC or memory IC is categorized not as a heater element, but as a temperature-restricted element.
In the composite IC device 100 shown in FIG. 9B, the resistor 41 as a representative of a heater element and the power transistor 51 as a representative of a temperature-restricted element are mounted on a single alumina (Al2O3) substrate 3. The lamp driving circuit shown in FIG. 9A is thus formed on the single alumina substrate 3.
The alumina substrate 3 having the heat element 41 and the temperature-restricted element 51 is attached, with an adhesive, on a seat member 2a of a lead frame 2 made of copper. Each pad 30 formed on the alumina substrate 3 and each lead pin 2b of the lead frame 2 are connected with each other by a bonding wire 6. Entire structure including the alumina substrate 3, the seat member 2a, and the wires 6 is molded and sealed with a mold resin 1 along with lead pins 2b being projecting, as shown in dashed lines of FIGS. 9B, 9C.
In this structure, heat generated from the heat element 41 is conducted to heat the temperature-restricted element 51, so that the temperature-restricted element 51 is apt to incur an operational defect. Consequently, as a countermeasure, applied voltage is restricted to suppress heat generation of the heater element 41, so that heat is inhibited from being conducted to the temperature-restricted element 51. However, since the applied voltage is restricted, electric current cannot be sufficiently supplied to the heater element 41.
As another countermeasure, the alumina substrate 3 is enlarged for dissipating heat so that heat conduction to the temperature-restricted element 51 can be suppressed. However, this countermeasure is against requirement of downsizing and the composite IC device 100 undesirably becomes larger. Furthermore, dissipating fins can be provided on the alumina substrate 3 to enhance dissipation ability and suppress heat from being conducted to the temperature-restricted element 51. However, this results in increasing manufacturing cost of the composite IC device 100.